By looking at the night sky, you could conclude that the number of visible stars is infinite. But its nothing more false, without a telescope we can see maximum 6,000 stars.
Ever since Greek’s astronomers — 2,000 years ago — magnitude system started: stars were classified according to their magnitude. They were divided into six magnitudes (measure of its brightness). In The first class there were included the brightest stars in the night sky, whereas the faintest were of sixth magnitude. Sixth grade is the limit of human visual perception. Nowadays, with the invention of the telescope and modern equipment, the scale has been extended to 21 levels of Magnitude.
A star of a specific magnitudine’ level is two and a half times brighter than a star one magnitude less.
The first magnitude includes about 22 of the brightest stars of which the brightest is Sirius. With a magnitude of 1.6, Sirius is about 1,000 times brighter than the faintest star we can see with the naked eye.
More examples: the brightest planet Venus varies in brightness and is about -4.4 magnitude at maximum brightness. The Moon is -12.7 magnitude at maximum brightness and the Sun is -26.75 magnitude.